1. Field of the Invention
The present invention relates to an impedance matching circuit for a multi-band radio frequency device, and further to such multi-band radio frequency device, and an impedance matching method.
More particularly, such impedance matching is done at sub-bands of a multi-band radio frequency signal in order to match impedances between various parts of the multi-band radio device such as between amplifier stages, between mixers and amplifier stages, or the like.
2. Description of the Related Art
U.S. Pat. No. 6,243,566 discloses impedance matching for a power amplifier in a dual band transmitter for a radiotelephone. For example, such a dual band transmitter radiotelephone can use the GSM system which operates at 900 MHz, and the DCS system, which is similar to GSM except that it operates at 1800 Mhz. In a two-stage power amplifier, an inter-stage matching circuit matches the impedances between a first stage and a second stage of the two-stage power amplifier. The inter-stage matching circuit optimizes the impedances at 900 Mhz or 1800 MHz depending on which transmission mode is in use. Two field effect transistors are used as power amplifier stages. Between the stages is a 15 pF capacitance, and at the source of the first stage is a small 3 nH inductance which is connected to a voltage source. A 2.7 pF capacitance is connected between the inductance and the voltage source. A 100 pF capacitance is also connected to the voltage source with a diode connected from the 1000 pF capacitance to ground. A 1.5 kΩ resistor with an input node is connected between the 1000 pF capacitor and the diode. When a voltage source is connected to the input node, the diode turns on and the 1000 pF capacitance dominates the impedance of the interstage matching circuit. The capacitance values are calculated so that 900 GSM signals from the first stage of the power amplifier are matched to the second stage when the input node is connected to a 2.7 V positive voltage source. When a zero, negative, or floating voltage source is connected to the input node, the 2.7 pF capacitance is connected to the input node, and the 2.7 pF capacitance and the 3 nH inductance dominate the impedance of the inter-stage matching circuit which matches the 1800 MHz signals to the second stage. Thus, a voltage applied to an input node, and a diode switch determine a GSM mode or DCS mode of the impedance matching circuit. The impedance matching network, is thus shared for receiving GSM of DCS sub-band signals.
Other systems used with multi-band radio frequency devices include NMT-450 operating at 450 MHz, AMPS and D-AMPS operating at 800 MHz, PCS operating at 1900 MHz, or still other systems.